Palo Verde (Parkinsonia microphylla)
Palo Verde is one of the most common trees of the Sonoran Desert and is found throughout southern Arizona and southeastern California. Starting in April, the Palo Verde produces a brite yellow flower that stands in contrast to it’s characteristic green trunk and branches. In late spring it is common to see the Palo Verde covered in yellow blossoms with a blanket of yellow at it’s base from dropped flowers. It is this fact that makes the Palo Verde a bit of an allergy enigma.
As a general principle, plants that produce conspicuous and fragrant flowers do so to attract insects such as bees to distribute their pollen. In addition, the pollen is typically heavy and sticky so that it sticks to the insects rather than being wasted blowing in the wind. Wind pollenated plans on the other hand produce copious amounts of light pollen that easily catches a ride in a breeze, and often finds it’s way to the nose and eyes of allergy sufferers. Palo Verde, although an insect pollenated tree, can cause allergy problems just because of the huge volume of flowers that fall to ground, dry, and then picked up by the wind
The olive tree is native to the Mediterranean, Asia, and Africa and has been cultivated for it’s fruit for thousands of years. It is a hardy, drought and disease resistant tree, well suited for the harsh conditions of the Sonoran desert. It is an evergreen tree which can grow to 30 ft and has an attractive gray, often gnarled and twisted trunk gaining character with age. Some trees are hundreds of years old. The olive tree produces a small, inconspicuous pale, white flower which is wind pollinated and produces volumes of airborne pollen in the spring. Olive tree pollen is one of the most potent and sensitizing of the allergenic plants of Arizona. Because of it’s association with severe springtime allergy symptoms, the city’s of Phoenix and Tucson have banned the planting of fruiting Olive trees since the 1960s. 
